High Temperature Low Pressure Carburizing with Prenitriding Process - The Economic Option for Vacuum Carburizing

The original idea of grain growth limitation by preliminary nitriding preceding low pressure carburizing (LPC) is presented as a useful option for the FineCarb (R) technology. This new process called PreNitLPC (R) enables high temperature carburizing up to 1050 degrees C for a variety of common carburizing steels without any adverse effects on both microstructural or performance. The shortening of carburizing time may result in a decrease in manufacturing costs as well as energy consumption from 5-50% depending on the case depth. This has been confirmed on the basis of several pilot industrial installations. The metallurgical background of PreNitLPC (R) has been discussed at the microstructural level. The mechanism of nanonitrides precipitation has been found as the effective way for intensive nucleation of austenite grains and for inhibition of grain boundary movement at significantly high temperatures. The results of comparative investigations of fatigue and impact strength as well as pitting resistance for traditional endothermic gas carburizing (ENDO), standard LPC and PreNitLPC (R) are presented. They confirm the efficiency of the pre-nitriding option in grain growth limitation for plain carbon and low alloying steels enabling high mechanical properties to be achieved in machine parts hardened using PreNitLPC (R). The problem of increased carburizing temperature on heat treatment distortion is also discussed. The results of comparative geometrical measurements show the possibility to keep the distortion level from the high temperature carburizing similar to that for traditional ENDO carburizing, although this requires additional optimization of the PreNitLPC (R) parameters.